Multi-stage evaporation of Cu2ZnSnS4 thin films

Multi-stage evaporation is a well-established method for the controlled growth of chalcopyrite thin films. To apply this technique to the deposition of Cu2ZnSnS4 thin films we investigated two different stage sequences: (A) using Cu2SnS3 as precursor to react with Zn-S and (B) using ZnS as precursor to react with Cu-Sn-S. Both Cu2SnS3 and ZnS are structurally related to Cu2ZnSnS4. In case (A) the formation of copper tin sulphide in the first stage was realized by depositing Mo/SnSx/CuS (1<x<2) and subsequent annealing. In the second stage ZnS was evaporated in excess at different substrate temperatures. We assign a significant drop of ZnS incorporation at elevated temperatures to a decrease of ZnS surface adhesion, which indicates a self-limited process with solely reactive adsorption of ZnS at high temperatures. In case (B) firstly ZnS was deposited at a substrate temperature of 150 degrees C. In the second stage Cu, Sn and S were evaporated simultaneously at varying substrate temperatures. At temperatures above 400 degrees C we find a strong decrease of Sn-incorporation and also a Zn-loss in the layers. The re-evaporation of elemental Zn has to be assumed. XRD measurements after KCN-etch on the layers prepared at 380 degrees C show for both sample types clearly kesterite, though an additional share of ZnS and Cu2SnS3 can not be excluded. SEM micrographs reveal that films of sample type B are denser and have larger crystallites than for sample type A, where the porous morphology of the tin sulphide precursor is still observable. Solar cells of these absorbers reached conversion efficiencies of 1.1% and open circuit voltages of up to 500 mV. (C) 2008 Elsevier B.V. All rights reserved.